| Summary: | Abstract. The Motzfeldt alkaline magmatic complex (MAMC) in southern Greenland is an interesting plutonic assemblage that may contain a valuable resource Nb-Ta-Zr-REE. It is a member of the set of alkaline intrusive rocks called the Gardar intrusions which occurred during a period of Proterozoic rifting (1,350–1,140 Ma). These includes also the prospective Ilímaussaq intrusion which is enriched in Rare Earth‐elements. The relationship to this deposit, plus the period of magmatism make the MAMC a potential prospect to be explored. At a regional scale, a geochemical dataset was used to determine the differences between three formations found in the MAMC. The normative mineralogy of the three formations show distinct variation but all three have a constant peralkalinity signature. This indicates that the three formations have a similar source composition and evolution. However, the oldest and middle formations underwent alteration that caused variation in their normative mineralogy. Elevated silica and trace element compositions related to the peralkaline rocks towards the edge of the MAMC indicates that mobilization of volatiles was achieved through the incorporation of country rock. Large scale circulation of fluids formed an ‘outer shell’ enriched in Nb-Ta-Zr-REE and SiO2 particularly during the intrusion of the mid Motzfeldt Sø formation. At a local scale, separate layers of an outcrop were sampled and analyzed to determine compositional variations between them. The samples were composed of syenites, microsyenites and pegmatites. A very late stage volatile rich fluid derived from the degassing of peralkaline microsyenites formed Nb-Zr-Ta-REE rich pegmatites at layer boundaries and caused fracturing at the microsyenite contacts. The host minerals for these elements, found in thin section of the pegmatites, include pyrochlore, zircon, thorite, eudialyte and bastnäsite. The samples have also been compared to other known REE deposits. The pegmatite samples are characterized by a low LREE/HREE ratio and a relatively ...
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